Apparatus for growing crystals



Sept. 14, 1965 A. l. BENNET'l j JR., ETAL 3,206,285

APPARATUS FOR GRbWING CRYSTALS 2 Sheets-Sheet 2 Filed July 23, 1959 TwinPlane Fig.5.

llllll 78 I H T r i E M m R LmB mm n7 United States Patent 3,206,286APPARATUS FOR GROWING CRYSTALS Allan I. Bennett, Jr., and Richard L.Longini, Pittsburgh, Pa., assignors to Westinghouse ElectricCorporation, East Pittsburgh, Pa., a corporation of Pennsylvania FiledJuly 23, 1959, Ser. No. 829,069 4 Claims. (Cl. 23-273) This inventionrelates to a continuous process for producing crystals of solidmaterials, and, in particular, to intrinsic and suitably dopedsemi-conductor crystals.

This invention is closely related to U.S. patent application, Serial No.757, 832, filed August 28, 1958 and now abandoned, of which one of thepresent inventors is the inventor, and assigned to the assignee of thepresent application.

At the present time, crystals of many solid materials are produced bypreparing a melt of the solid material, contacting the surface of themelt with a previously prepared crystal of the material and slowlywithdrawing the previously prepared crystal, usually at a rate of theorder of an inch an hour to produce a desried grown crystal member. Ithas been the invariable practice in this procedure to maintain the meltduring crystal growing at a temperature slightly above the melting pointof solid material.

The nature and configuration of the withdrawn crystals produced by suchprior art practices have generally been uncontrollable except withinrelatively broad limits. Thus, the thickness has not been radilymaintained within precise dimensions. In many cases, surface andinternal perfections such as dislocations and other crystal structureflaws have been present in the grown crystals.

In the semiconductor industry, crystals of silicon, germanium, andcompounds of the Group III-Group V elements of the Periodic Table havebeen grown from melts in accordance with this prior art practice. Inorder to employ such grown crystals in semiconductor devices, it hasbeen necessary to saw them into slices using, for example, diamond saws.Thereafter, dice of desired shape have been cut from these slices. Thesawed surfaces of the dice have been lapped or otherwise mechanicallypolished to remove disturbances or otherwise unsatisfactory surfacelayers, which treatment is followed by an etch to remove microscopicsurface imperfections. As a result of this working, which is performedon expensive precision machinery and requires highly-skilled labor,there may be a loss of as much as 90% of the original grown crystals insecuring dice which have useful semi conductor shape and configuration.

An object of the present invention is to provide apparatus forcontinuously producing intrinsic or suitably doped crystals of anydesired length, precisely controlled thickness and width from a suitablesupercooled melt ,of a material.

Another object of the present invention is to provide apparatus forcontinuously producing intrinsic or suitably doped fiat dendriticcrystals of materials having a diamond cubic lattice structure with flatfaces having precise (111) surfaces.

Another object of the present invention is to provide a continuousprocess for producing intrinsic or suitably doped crystals of anydesired length, precisely controllable thickness and width fromsupercooled melts of solid materials.

Another object of the invention is to provide a continuous process forgrowing intrinsic or suitably doped fiat dendritic crystals from asuitable supercooled melt while maintaining low temperature gradients inthe crystals above the melt surface so that imperfection in the growingcrystals are minimized.

"ice

Another object of the invention is to continuously produce intrinsic orsuitably doped fiat dendritic crystals of materials having a diamondcubic lattice structure with flat faces having (111) surfaces.

Other objects of the invention will, in part, be obvious and will, inpart, appear hereinafter.

For a better understanding of the nature and objects of the invention,reference should be had to the following detailed description anddrawings, in which:

FIGURE 1 is a view in elevation, partly in cross section, of acontinuous crystal growing apparatus in accordance with the teachings ofthis invention;

FIGURE 2 is a greatly enlarged fragmentary view of a dendritic crystal;

FIG. 3 is a side view partially in cross-section of two aligning rollerswith a crystal disposed centrally thereon;

FIG. 4 is a side view of a drive system suitable for use in accordancewith the teachings and apparatus of this invention; and,

FIGS. 5 and 6 are two top views of drive systems suitable for use inaccordance with the teachings of this invention.

In accordance with the present invention, it has been discovered thateither intrinsic or doped crystals of solid materials may becontinuously prepared as flat dendritic crystals of continuous orinfinite length having a closely controlled thickness with relativelyprecise fiat parallel faces. These flat dendritic crystals may becontinuously pulled or grown from suitable melts at a relatively highrate of speed of pulling of the order of times greater than the linearpulling velocities previously employed in the art. The thickness of thecrystals may be readily controlled and surface imperfections minimizedor reduced by following the teachings of the present invention.

More particularly, in practicing the process of this invention in itspreferred embodiment, a melt comprised of either the intrinsic or purematerial to be grown into a flat dendritic crystal or the materialcombined with at least one selected doping material is prepared at atemperature slightly above the melting temperature thereof. The surfaceof the melt is contacted with a previously prepared crystal having asingle twin plane at the interior thereof, the crystal being orientedwith the 2ll direction vertical to the melt surface. Other necessary ordesirable crystallographic and physical features of the seed crystalwill be pointed out in detail hereinafter. The seed crystal is dippedinto the surface of the melt a suf ficient period of time to causewetting of the lower surface of the seed, usually a period of time of afew seconds to a minute is adequate and, then, the melt is supercooledrapidly, following which the seed crystal is withdrawn with respect tothe melt at the speed of the order of from 1 to 10 inches a minute.Under some conditions, considerably slower pulling speeds can beemployed, for example, 0.2 inch per minute. Pulling speeds of from 10 to25 inches per minute have given good results. The degree ofsupercooling, the rate of pulling of the seed crystal from the melt canbe so correlated so as to produce a thin strip of solidified meltmaterial of continuous or infinite length having a precise desiredthickness and with, doping concentration, and having the desiredcrystallographic orientation.

The present invention is particularly applicable to solid materialscrystallizing in the diamond cubic lattice structure or zinc blendstructure. Two examples of such materials are the elements silicon andgermanium. Likewise, stoichiometric compounds having an average of 4valence electrons per atom respond satisfactorily to the continuouscrystal growing process of this invention. Such compounds, which may becontinuously produced with excellent results, comprise substantiallyequal molar proportions of an element from Group III of the PeriodicTable particularly gallium and indium. Combined with an element fromGroup V of the Periodic Table, particularly phosphorus, arsenic andantimony. Those materials crystallizing in the diamond cubic latticestructure are particularly satisfactory for various semiconductor orother applications.

Furthermore, the diamond cubic lattice structure materials may beintrinsic, or, as stated before, may be doped with one or moreimpurities to produce n-type or p-type semiconductor materials, Forinstance, the materials may be grown from a melt containing both nandp-type doping materials, whereby the pulled or grown crystal will havealternate layers of pand n-type semiconductivity. The melt may alsocontain both an nand p-type dopant but in such quantities that one willprevail over the other throughout the grown crystal. The continuouslayer growing process and apparatus of the present invention may beapplied to all of these different materials.

For a better understanding of the practice of this invention, referenceshould be had to FIG. 1 of the drawing, wherein there is illustratedapparatus for continuously producing dendritic crystals of indefinitelength in accordance with the teachings of this invention. The apparatuscomprises a base 12 carrying a support 14 for a crucible 16 of asuitable refractory material such as graphite to hold a melt 18comprised of the material from which flat dendritic crystals are to becontinuously drawn. Molten material 18, for example, germanium, is maintained within the crucible 16 in the molten state by a suitable heatingmeans such, for example, as an induction heating coil 20 disposed aboutthe crucible. Controls, not shown, are employed to supply alternatingelectrical current to the induction coil 20 to maintain a closelycontrollable temperature in the body of the melt 18. The temperatureshould be readily controllable to provide a temperature in the melt afew degrees above the melting point and also to reduce heat inputs sothat the temperature drops in a few seconds, for example, in 5 toseconds, to a temperature at least one degree below the meltingtemperature and preferably to supercool the melt from 5 to 15 C. orlower. A cover 22 closely fitting the top of the crucible 16 may beprovided in order to maintain a low thermal gradient above the top ofthe melt.

A second crucible 26 of a suitable refractory material such as graphiteis supported on base 12 by a support member 29. The crucible 26 containsan additional quantity of melt 18 which may be charged in the crucible16 through a suitable conduit 28 by an actuating valve 3% operable byelectromagnetic controls (not shown) or by other suitable means. Theconduit 28 is preferably attached to crucible 16 to supply moltenmaterial at the bottom of the charge 18 therein so that it can attainthe desired temperature before reaching the surface 19. The crucible 26has a top member 32 and is surrounded by a heating means, such, forexample, as an induction heating coil 34. Controls, not shown, areemployed to supply alternating electrical current to the induction coil34 to maintain a closely controllable temperature in the melt 18 withincrucible 26. The temperature of the melt within crucible 26 should be afew degrees above the melting point. The melt within crucible 26 is usedto replenish the melt in crucible 16. While crucible 26 has beenillustrated in FIG. 1 in a particular relation to crucible 16, it willbe understood that crucible 26 may be disposed anywhere within theapparatus 10. It will also be understood that depending upon the lengthof dendrite to be grown and the capacity of crucible 16, it may not benecessary to employ an auxiliary crucible such as crucible 26.

An aperture 24 is provided in the cover 22 of crucible 16 through whicha seed crystal 44 may pass and through which a dendrite is to be pulled.

Referring to FIG. 2 of the drawing, there is illustrated in greatlyenlarged view, a section of a preferred seed crystal 44 having a singletwin plane. Such seed crystals may be obtained in various ways, forexample, by supercooling a melt of the solid material to a temperatureat which a portion thereof solidifies, at which time some dendriticcrystals having a single internal twin plane will be formed and may beremoved from the melt. While these crystals may not be uniform, they aresuitable for seed purposes. Also, one can cut from a large twin crystala section suitable for use as a seed crystal.

The seed crystal 44 comprises two relatively fiat parallel faces 45 and47 with an intermediate interior twin plane 49. The twin planeordinarily will be precisely between the faces 45 and 47. Examinationwill show that the crystallographic structure of the preferred seed onboth faces 45 and 47 is that indicated by the crystallographic directionarrows at the right and left faces respectively of the figure. It willbe noted that the horizontal direction perpendicular to the flat faces45 and 47 and parallel to the melt surfaces are 111 The direction ofgrowth of the dendritic crystal will be in 2l1 crystallographicdirection. If the faces 45 and 47 of the dendritic crystal 44 were to beetched preferentially to the (111) planes, they will both exhibitequilateral triangular etch pits 51 which vertices 53 will pointupwardly while their bases will be parallel to the surface of the melt.It is an important feature of the preferred embodiment of the presentinvention that the etch pits on both faces 45 and 47 of seed crystal 44have their vertices 53 pointing upwardly. A non-twinned crystal orcrystal containing two twin planes or any even number thereof willexhibit triangular etched pits on one face whose vertices will bepointing opposite to the direction of the vertices on the other face.The most satisfactory crystal growth can only be obtained by employingseed crystals of the type exhibited in FIGURE 2 wherein only a singletwin plane is present interiorly.

Seed crystals having an odd number of twin planes containing the growthdirection may be employed in practicing the process of this invention,due care being had to point the triangular etch pits on the outer faceof the crystal with their vertices upwardly and the bases parallel tothe surface of the melt. Further, seed crystals containing an evennumber of twin planes may be employed for crystal pulling, though asdesirable pulled crystals will not be obtainable as with the preferredsingle twin plane seed crystal shown in FIG. 2.

The direction of withdrawal of seed crystal 44 having a single twinplane from the melt 1 8 must be with the direction of the vertices 53 ofthe etched pits being upward and bases being substantially parallel tothe surface of the melt. When so withdrawn, the melt will solidify atthe bottom of the crystal in a satisfactory prolongation thereof. If thecrystal 44 were to be inserted into the melt so that the vertices 53 arepointed downwardly, very erratic grown crystals will be produced whichare not only nonuniform in direction but grow at angles of to thelongitudinal axis of the seed and produce very irregular spines andlateral projections, and generally are unsatisfactory.

Referring again to FIG. 1, a protective enclosure 36 of glass, quartz orother suitable material is disposed about the crucibles 16 and 26 with acover 38 closing the top thereof.

Within the interior of enclosure 36 there is provided a suitableprotective atmosphere entering through a conduit 40 and, if necessary, avent 42 may be provided for circulating a current of such protectiveatmosphere. Depending on the crystal material being processed in theapparatus, the protective atmosphere may comprise a noble gas, such ashelium or argon, or a reducing gas such as hydrogen or mixtures ofhydrogen and nitrogen, or nitrogen or the like or mixtures of two ormore gases. In some cases, the interior of apparatus 10 may be evacuatedto a high vacuum in order to produce crystals of materials free from anygases.

In the event that the process is applied to compounds having onecomponent with a high vapor pressure at the temperature of the melt, aseparately heated vessel containing the component may be disposed in theenclosure 36 to maintain therein a vapor of such compound at a partialpressure suflicient to prevent impoverishing the melt or the growncrystals with respect to the component. Thus, an atmosphere of arsenicmay be provided when doped or intrinsic crystals of gallium arsenide arebeing pulled. The enclosure 36 may be suitably heated, for example, byan electrically heated blanket or wrapping to maintain the walls thereofat a temperature of the separately heated vessel containing the arsenicin order to prevent condensation of arsenic therein.

In operation of the apparatus 10, the seed crystal 44 is passed throughthe aperture 24 in the cover 22 of crucible 16. At the beginning of theoperation, the seed crystal 44 is butt soldered or otherwise joined toone end of a leader tape 46. The tape 46 may be of steel or any othersuitable material or may be comprised of a previously grown dendrite. Itis preferable but not necessary that the tape 46 have the same width andthickness as that of the dendrite to be grown. The other end of the tape46 is passed between guide rollers 48, 50, 52 and 54, and is fastened toa winding drum 68. The seed crystal 44 is lowered into crucible 16 untilit contacts the surface 19 of the melt 18. The melt 18, which isslightly above its melting point, dissolves the tip of the seed crystal.A meniscus-like contact between the molten tip of the seed crystal andthe melt is formed.

The power input to the heating coil 20 is reduced in order to supercoolthe melt (or reducing the applied heat if other modes of heatapplication than inductive heating are employed). There will be observedin a period of time of the order of 5 seconds after the heat input isreduced to a crucible of about two inches in diameter and two inches inlength, the supercooling being about 8 C., an initial elongatedhexagon-a1 growth or enlargement on the surface of the melt adjoiningthe tip of the seed crystal. The hexagonal surface growth increases inan area so that in approximately 10 seconds after heat input is reduced,its area is approximately three times that of the cross section of theseed crystal. At this stage, there will be evident spikes growing out ofthe two opposite hex-- 'agonal vertioes lying in the plane of the seed.These spikes appear to grow at a rate of approximately two millimetersper second. When the spikes are from two to three millimeters in length,the seed crystal pulling mechanism, which will be discussed in detailhereinafter, is energized to pull the crystal from the melt at thedesired rate. The initiation of pulling is timed to the appearance andgrowth of the spikes for best results.

After pulling the seed crystal upwardly from the supercooled melt, itwill be observed that the fiat solid diamond shaped area portion isattached to the seed crystal and that a downwardly extending dendriticcrystal has formed at each end of the elongated diamond area adjacent tothe spike. Accordingly, two dendritic crystals can be readily pulledfrom the melt at one time from a single seed crystal. By continuedpulling, the two dendritic crystals may be extended to any desiredlength.

If the seed crystal is disposed so that one edge is nearer the thermalcenter of the melt crucible than the other edge, it is possible toincrease briefly either the pulling rate or the temperature of the melt,and under these variations the dendritic crystal furthest away from thethermal center or any hotter region will usually stop growing andthereafter only a single dendritic crystal will be attached to and growfrom the seed. Also, one of the dendrites can be mechanically severedfrom the seed.

For the purpose of this invention, it is preferred to pull only a singledendrite from the melt. The control of the width and thickness poseproblems that can be best handled with a single dendrite.

If the double dendritic crystal attached to the original seed crystal isintroduced into the same or another melt slightly above the meltingtemperature and after supercooling the melt, on pulling the doubledendritic crystal from the surface, there will be formed two diamondshaped areas attached to the double dendrite and four dendritic crystalswill be pulledtwo attached to each of the original dendrites. Thus, inone instance four germanium dendrites in length were pulled from a melt.While more than four dendritic crystals can be pulled from the melt,there may arise interference and other factors which will render suchgrowth difficult. However, it is to be understood that the pulling ofmore than one dendrite from one seed may be extremely diflicult andordinarily is not desired.

A number of methods for pulling only a single dendrite from a seed areavailable and may be employed in practicing the invention.

The seed crystal 44 is pulled from the melt 18 by activation of rollers48, 50, 52 and 54. The rollers 48, 50, 52 and 54 are comprised of amaterial that will not scratch the dendrite, but will grip the materialtightly enough to prevent slippage, for example neoprene rubbers, nylon,polytetrafluoroethylene, polytrifluoromonochloroethylene and the like.

During growth, unless the dendrite pull and growth direction is exactlyperpendicular to the axis of the drum 60, the growing dendrite will notremain centered upon the drum but will move axially along it. Thecontemplated length of pull, which may be several hundreds of feet, isenough that a very minute misalignment will eventually result in anintolerable axial displacement of the dendrite upon the drum. Suchmisalignment will result also from errors in the initial alignment ofthe seed in the melt. Small differences in the physical shape and sizeof the dendrite may result in axial displacements of undesirablemagnitude. It is therefore necessary that the lateral position of thedendrite be controllably aligned in some manner. The rollers 48, 50, 52and 54 serve to align the dendrite. The rollers are so designed that theonly stable position of the dendrite is at the center of the rollers.

With reference to FIG. 3, there is illustrated a sectional view of therollers 48 and 50 with a dendrite 44 aligned centrally thereon. Eachroller has a riged shaft disposed therethrough and connected to adriving means which will be described hereinafter. If the shafts werecoaxial, the sensing characteristic of the rollers would be lost. Theaxes of the rollers 48 and 50 and S2 and 54 are slightly angularlydisplaced with respect to each other, for example, each axis is inclinedapproximately one-half degree above the horizontal. The rollers 48 and50 and 52 and 54 are butted together with sufficient axial pressure tomaintain contact over the entire end faces 55 and 58 respectively of therollers. As a consequence of this arrangement, a dendrite passed upwardbetween rollers 48, 5t) and 52 and 54 will be stable in a lateralposition only at the center division line thereof at 55 and 58.

The pulling of the dendrite 44 from the melt 18 may be accomplished byapplying a driving force to both the rollers 48, 50, 52 and 54 and todrum 60 or by allowing the rollers, or one set of rollers, to idle andapplying the main pulling force to only one set of rollers and/or onlyto the drum 60. The rollers and drum are maintained at the controlledspeed necessary to maintain the desired crystal pull rate.

With reference to FIG. 4 there is illustrated one suitable system forpulling the dendrite by driving only one set of rollers. A separatemeans is used to drive the drum. The system will be described in termsof driving rollers 52 and 54 with rollers 48 and 50 idling. It will beunderstood of course that rollers 48 and 50 may be driven with 52 and 54idling. In the system illustrated in FIG. 4, power is transmitted froman electric motor 62 to a shaft 64. Two gears 66 and 68 are disposedupon shaft 64. The gears 66 and 68 drive two hypoid gears 7 70 and 72which in turn drive shafts 63 and 61 to which are connected rollers 54.and 52. A duplicate identical system could be used to drive rollers 48and 50 at the same time.

With reference to FIG. 5, there is illustrated one suitable drive systemfor driving all four rollers. In this system, the angular speed of thefour rollers will be equal but the torque upon the different rollers mayvary. An electric motor 74 is connected to a gear 76 through shaft 78.The gear 76, is meshed with and drives gear 80 which in turn meshes withand drives gear 82. Gear 80 through shaft 84 and gears 86 and 88 drivesshaft 59 which is connected to roller 50 (not shown). Gear 80 throughshaft 84 and gears 90 and 92 drives shaft 63 connected to roller 54.Gear 82 through shaft 94 drives gears 96 and 98 which drive shaft 57connected to roller 48 and; through gears 100 and 102 drives shaft 61which is connected to roller 52.

With reference to FIG. 6, there is illustrated a second satisfactorymethod for driving all four rollers. The system illustrated in FIG. 6will drive the rollers in such a manner that the torque will be the sameon all four rollers but the speed of the rollers may differ. In thesystem of FIG. 6 an electric motor 104 drives a shaft 106 which in turndrives a gear 108. Gear 108 drives a gear 110 in a plane perpendicularto shaft 106. Gear 110 drives gear 112 which drives gears 114 and 116.Gear 114 drives gear 118 which in turn drives shaft 59 which isconnected to roller 50. Roller 54 is driven in a like manner throughshaft 63 and gear 120. A duplicate system is used to drive rollers 48and 52.

As explained above herein, the rollers 48, 50, 52 and 54 may be idlerollers and all the pulling force on the dendrite is supplied by thedrum 60. Since the radius of the drum 60 is changing constantly as thedendrite is wound thereon, the motor driving the drum should be aconstant torque variable speed motor.

Even when the crystal pulling force is supplied by the rollers aconstant torque force must be applied to the drum to maintain a tensionon the segment of the dendrite between the rollers and the drum. If thistension is not maintained the dendrite may sag into a loop and break.

Referring again to FIG. 1, in addition to aligning the dendrite by theuse of rollers such as described hereinabove, the position of thedendrite may be sensed by photoelectrical or optical means 5. Thedendrites have essentially perfect, optically flat surfaces that areparallel to each other within a few angstroms. Consequently, opticalmeans based on interference techniques may be employed to determine therelative thickness of the dendrites and to sense any changes inthickness. Thus, a beam of monochromatic light may be split and one partdirected on one surface and the other part directed on the othersurface. The beams are reflected from the surfaces and brought togetherto indicate by the interference fringe changes in relative thickness ofthe dendrite. Known control devices (not shown) operating oninterference measurements are correlated with the optical device toindicate the thickness as well as changes in thickness, and further tovary the pull rate of the dendrite and the heat input to the meltthrough electrical leads 7 and 9 respectively.

In addition, guide arms straddling the dendrite can be activated by amotor responsive to the photoelectrical. or optical sensing means toshift the dendrite and thus it may be kept aligned in a desiredposition.

After passing between rollers 48 and 50 and 52 and 54, the dendrite 44is wound around the drum 60. To prevent the dendrite from beingscratched as it is'rolled onto roller 60, a plastic interlayer, forexample, a film, foil or tape of polyethylene, polytetrafluoroethylene,polytrifluoromonochloroethylene, nylon and the like may be disposedabout or between each succeeding layer of dendrite. The plastic stripmay be formed with a flat depression in the middle of either or bothsides to act as a spacer. The

8 plastic strip will also serve to keep the dendrite in place if thereshould be a break during the drawing process.

Because of the thermodynamic relationship between the supercooled meltand the growing crystal, the pull rate and the thickness of the growncrystal and other factors when a dendrite of considerable length isbeing pulled there is a tendency for the grown crystal to progressivelygrow thicker. This condition can be overcome with a rather suddendisplacement or jerking of the dendrite crystal with respect to the meltin a vertical upward direction. The sudden displacement causes thedendrite to decrease in thickness a fraction of a mil. Repeating thesudden displacement after a minute or so, another decrease in thicknesswill be effected. This may be repeated as often as is necessary toproduce or maintain a desired thickness of dendrite.

Such vertical displacement or jerking can be accomplished for example byimparting a saw-tooth displacing movement to the crucible. The cruciblemay be dropped rapidly, for example, 2 or 3 mm., and then raised over aperiod of about 30 seconds back to its normal position. The procedure isrepeated as necessary. The same result may be accomplished by suddenlydisplacing the entire pulling mechanism in an upward direction and thengradually lowering it to its initial position over a period of 30seconds or so. The procedure is repeated as necessary. Anotheralternative would be to jerk the winding drum in an upward direction andthen slowly lower it. The mentioned range of 2 to 3 mm. is intended onlyas an illustrative example, as the actual distance will depend upon thethermal gradient between melt and crystal, rate of pulling, and degreeof supercooling of the surface of the melt. Likewise, the 30 secondperiod for returning the displaced member of the system to its normalposition is intended merely as an example, the important factor beingthat the return rate to the original position is lower than the pullrate of the crystal from the melt.

In a modification of the present invention, instead of rolling the drawndendrite about a drum such as drum 60 illustrated in FIG. 1, the growndendrite may be passed directly into an assembly line whereby it isprocessed into a semiconductor device or series of devices by beingacted upon by suitable doping materials and having electrical contactsor leads attached thereto. The contacts, leads and doping materials maybe applied in accordance with the teachings of US. application SerialNo. 807,570, of A. I. Bennett, Jr., R. L. Longini and H. F. John (W. E.Case 32,096), filed April 20, 1959, US. Patent No. 3,106,764, theassignee of which is the same as that of the present invention.

In a further modification, the grown dendrite strip may be cut intosections after passing through rollers 48, 50, 52 and 54, rather thanbeing deposited upon a drum. If it is desired to cut the grown dendriteinto strips of a predetermined length within the apparatus 10, it isimportant that the cutting be done in such a manner that no fragmentsmay fall back into the melt 18 disposed Within crucible 16.

In accordance with another modification of this present invention, thedendrite strip 44 may be grown with predetermined p-n-p or n-p-n regionstherein in accordance with the teachings of application Serial No.824,355, of A. I. Bennett, Jr. (W. E. Case 31,877), filed July 1, 1959,and now abandoned, the assignee of which is the same as in the presentinvention.

The following examples are illustrative of the practice of thisinvention.

Example I In apparatus similar to FIG. 1, a graphite crucible containinga quantity of intrinsic germanium is heated by the induction coil to atemperature several degrees above the melting point of germanium, thetemperature being about 938 C., until the entire quantity forms a moltenpool. A dendritic seed crystal having a single interior plane andoriented as in FIG. 2 of the drawing is soldered to a thin steel strip.The seed crystal is passed throughthe aperture in the top of thecrucible until its lower end touched the surface of the moltengermanium. The contact with the molten germanium is maintained until asmall portion of the end of the dendritic seed crystal had melted.Thereafter, the temperature of the melt is lowered rapidly in a matterof five seconds by reducing current to the coil 20, to a temperature 8below the melting point of the germanium so that the melt is supercooled(about 928 C.). After an interval of approximately ten seconds, at thistemperature, the germanium seed crystal is pulled upwardly at a rate ofapproximately seven inches per minute. The pulling is accomplished bytwo sets of rollers as illustrated in FIG. 1. The two sets of /2 inchdiameter rollers turning in opposite directions at a speed ofapproximately 4 r.p.m. The steel strip and the dendritic crystal iswound on a drum. A thin film of polyethylene is disposed between eachwinding of the dendritic on the drum. The dendrite can be of indefinitelength providing the germanium in the crucible is replenished.

The dendritic crystal thus prepared will have a thickness of 7 mils anda width of approximately 2 millimeters. The grown dendritic crystal willhave substantially fiat and highly parallel faces from end to end with(111) orientation. The germanium dendritic crystal so grown will befound to have no surface imperfections except for a number ofmicroscopic steps difiering by about 50 angstroms and will be of aquality suitable for semiconductor applications.

Example II The process of Example I is repeated except for increasingthe pull rate to 12 inches per minute. The dendritic crystal isapproximately 3.5 mils in thickness and of a width of about 30 mils. Thesurface perfection and flatness is exceptional.

Example 111 Example IV A melt of indium antimonide is prepared followingthe procedure of Example I employing apparatus as illustrated in FIG. 1of the drawing. The indium antimonide is withdrawn at a rate of inchesper minute from a melt supercooled 5 C. The resultant flat dendriticcrystal is tested and found to be suitable for semiconductorapplications. The surface had (111) orientation.

Example V The procedure of Example I is repeated evcept that the melt iscomprised of a quantity of germanium and 1 10 by weight, antimony and 110 by weight, boron. The crystal is pulled at a rate of 7 inches perminute. The resultant dendritic crystal has substantially fiat highlyparallel faces from end to end with (111) orientation. The germaniumdendritic crystal so grown is found to have n-p-n alternate regionstherein. The crystal so prepared is suitable for fabrication into asemiconductor device merely by attaching of leads thereto.

While emphasis has been made herein with respect to semiconductormaterials, it will be understood that the apparatus and process can beapplied to materials otherwise meeting the requirements set forth hereinbut are not considered to be semiconductors.

While the invention has been described with reference to particularembodiments and examples, it will be understood, of course, thatmodifications, substitutions and the like may be made therein withoutdeparting from its scope.

We claim as our invention:

1. In apparatus for growing thin fiat dendritic crystals of any desiredlength, in combination, a crucible containing a confined melt of amaterial from which the dendrite is to be grown, electrical heatingmeans associated with the crucible for maintaining a predeterminedtemperature Within the melt, and roller means disposed above thecrucible, said roller means being comprised of two pairs of rollers,each pair consisting of two rollers having an axis, and a rigid shaftpassing through said axis, the axes of each roller being angularlydisplaced with respect to each other and inclined above the horizontal,the rollers of each pair having end faces butted together withsuflicient axial pressure to maintain contact over the entire end facesof two adjacent rollers, said pairs of rollers disposed in a generallyhorizontal plane and in a relationship to engage a withdrawn dendrite,said roller means being capable of pulling the dendritic crystal fromthe melt and maintaining said crystal in a relatively fixed positionrelative to said melt.

2. In apparatus for growing thin flat dendritic crystals of any desiredlength, in combination, a crucible containing a confined melt of amaterial from which the dendrite is to be grown, electrical heatingmeans associated with the crucible for maintaining a predeterminedtemperature within the melt, roller means disposed above the crucible,said roller means being comprised of two pairs of rollers, each pairconsisting of two rollers having an axis, and a rigid shaft passingthrough said axis, the axes of each roller being angularly displacedwith respect to each other and inclined above the horizontal, therollers of each pair having end faces butted together with sufiicientaxial pressure to maintain contact over the entire end faces of twoadjacent rollers, said pairs of rollers disposed in a generallyhorizontal plane and in a relationship to engage a withdrawn dendrite,said roller means being capable of pulling the dendritic crystal fromthe melt and maintaining said crystal in a relatively fixed positionrelative to said melt, winding means capable of receiving said dendriticcrystal disposed above the melt, and means for replenishing the melt asdendritic crystals are withdrawn from the melt.

3. In apparatus for growing thin fiat dendritic crystals of any desiredlength, in combination, a crucible containing a confined melt of amaterial from which the dendrite is to be grown, electrical heatingmeans associated with the crucible for maintaining a predeterminedtemperature within the melt contained within the crucible, roller meansdisposed above the melt, said roller means including aligning means sothat it is capable of pulling the dendritic crystal from the melt andmaintaining said crystal in a relatively fixed position relative to saidmelt, means responsive to the dimensions of the dendritic crystal tocontrol the temperature of the melt and rate of pulling, and means forreplenishing the melt as dendritic crystals are withdrawn from the melt.

4. In apparatus for growing thin flat dendritic crystals of any desiredlength, in combination, a crucible containing a confined melt of amaterial from which the denrite is to be grown, electrical heating meansassociated with the crucible for maintaining a predetermined temperaturewithin the melt contained within the crucible, roller means disposedabove the melt, said roller means being capable of pulling the dendriticcrystal from the melt and maintaining said crystal in a relatively fixedposition relative to said melt and means for imparting a temporarysudden separation of the order of 1 mm. to 5 mm. between the melt andthe dendritic crystal, whereby, the thickness of the dendrite isdecreased, means for returning the dendritic crystal with respect to themelt at a slow rate, and

means for replenishing the melt as dendritic crystals are withdrawn fromthe melt.

References Cited by the Examiner UNITED STATES PATENTS Strong. Ritzmann.Himmelheber et al. Koury 148-15 Kniepkamp 23301 Shockley 148-1.5Schweickert et a1. 23273 12 2,907,643 10/59 Reynolds et a1. 232732,916,593 12/59 Herrick. 2,960,418 11/60 Zierdt 1481.5 2,993,301 7/61Muller 4983.1

OTHER REFERENCES Proceedings of the Royal Society; vol. 229, 1955, byBillig; pages 346-363.

10 NORMAN YUDKOFF, Primary Examiner.

ANTHONY SCIAMANNA, MAURICE A. BRINDISI,

RAY K. WINDHAM, Examiners.

1. IN APPARATUS FOR GROWING THIN FLAT DENDRITIC CRYSTALS OF ANY DESIREDLENGTH, IN COMBINATION, A CRUCIBLE CONTAINING A CONFINED MELT OF AMATERIAL FROM WHICH THE DENDRITE IS TO BE GROWN, ELECTRICAL HEATINGMEANS ASSICIATED WITH THE CRUCIBLE FOR MAINTAINING A PREDETERMINEDTEMPERATURE WITHIN THE MELT, AND ROLLER MEANS DISPOSED ABOVE THECRUCIBLE, SAID ROLLER MEANS BEING COMPRISED OF TWO PAIRS OF ROLLERS,EACH PAIR CONSISTING OF TWO ROLLERS HAVING AN AXIS, AND A RIGID SHAFTPASSING THROUGH SAID AXIS, THE AXES OF EACH ROLLER BEING ANGULARYLYDISPLACED WITH RESPECT TO EACH OTHER AND INCLINED ABOVE THE HORIZONTAL,THE ROLLERS